DESCRIPTION: (Applicant's Abstract) The use of antibody-targeted toxins and other effector molecules for the treatment of disease is presently constrained by collateral effects on normal tissues. Such undesirable action is due to non-specific uptake of circulating antibody-effector effector conjugates and/or low-level antigen expression by normal tissues. This project tests the hypothesis that a binary system for effector delivery will circumvent these problems. The essence of this binary strategy is to separately deliver the complimentary functional domains of an effector molecule to two distinct antigens on the surface of target cells. Diphtheria toxin is a prime example of a potent cytoactive moiety with essential component parts. Although virtually nontoxic when segregated, the catalytic and translocation domains become lethal if they recombine within a target cell. Therefore, complimentary domains will be targeted to different cell-surface antigens such as c-erbB-2, EGF receptor or transferrin receptor using bispecific antibodies. Potent cytotoxicity will result only if both domains are delivered to the same target cell and are released into the endosome. Consequently, cells that express both antigens are killed, but normal cells that may express only one of these antigens remain unaffected, greatly enhancing the specificity of targeting. Reagents with dual specificity were constructed by coupling antibodies against a selected cell-surface antigen to pH sensitive antibodies that bind one of the functional domains of diphtheria toxin. Using these constructs, the complimentary domains of diphtheria toxin can be delivered to a target cell and are rapidly released once the complex is internalized and exposed to low endosomal pH. A minimally sized, effective binary toxin will be defined by genetically engineering truncated domains of diphtheria toxin. The most promising binary toxins will be tested for distinctive therapeutic activity in vivo using a human xenograft model in mice.